The invention relates generally to data transmission in a telecommunication system, and particularly to data transmission in which the maximum transmission capacity of a traffic channel is as high as or only slightly higher than one user data rate at a terminal interface.
Mobile systems generally mean different telecommunication systems that enable private wireless data transmission for subscribers moving within the system. A typical mobile system is a public land mobile network PLMN. The PLMN comprises fixed radio stations (base stations) located in the service area of a mobile network, the radio coverage areas (cells) of the base stations providing a uniform cellular network. A base station provides a radio interface (air interface) in the cell for communication between a mobile station and the PLMN.
Another field of mobile systems includes satellite-based mobile services. In a satellite system, radio coverage is obtained by satellites instead of terrestrial base stations, the satellites being in orbit round the earth and transmitting radio signals between mobile stations (or user terminals UT) and land earth stations (LES).
Subscriber mobility requires similar solutions in satellite mobile systems as in the PLMNs, i.e. subscriber data management, authentication and location management of mobile subscribers, handover, etc. The satellite systems should also support similar services as the PLMNs.
One way of meeting the above requirements in satellite mobile systems is to use existing PLMN solutions. In principle this alternative is very straightforward since a satellite system can basically be compared to a base station system of a mobile system having a different radio interface. In other words, it is possible to use conventional PLMN infrastructure where the base station system(s) is(are) a satellite system. In such a case, the same network infrastructure could, in principle, even contain both conventional PLMN base station systems and satellite xe2x80x98base station systemsxe2x80x99.
There are many practical problems, however, in adaptation of the PLMN infrastructure and a satellite system. A problem apparent to the Applicant is that a PLMN traffic channel and a traffic channel of a xe2x80x98radio interfacexe2x80x99 in a satellite system differ considerably. Let us examine an example where the PLMN is the Pan-European digital mobile system GSM (Global System for Mobile Communication), and the satellite mobile system is the Inmarsat-P system that is currently being developed.
At present, a GSM traffic channel supports data transmission at user rates 2400, 4800, 7200 and 9600 bit/s. In addition to user data, status information on the terminal interface (control signals of a V.24 connection) is transmitted in both directions on the traffic channel. In transparent HSCSD data service, it is also necessary to transmit synchronization information between subchannels. In synchronous transparent bearer services, the clocking information of network independent clocking NIC must also be transmitted through a transmission channel from a transmitting terminal equipment to a receiving terminal equipment via a transmission network, when the transmission network and the transmitting terminal equipment are not in sync with each other, i.e. the terminal equipment uses network independent clocking (e.g. internal clock). The above-mentioned additional information raises the bit rate at the radio interface to be higher than the actual user rate. The GSM radio interface rates corresponding to user rates 2400, 4800 and 9600 bit/s are 3600, 6000 and 12000 bit/s. These signals are subjected to different channel coding operations, which raise the final bit rate to about 22 kbit/s.
The Inmarsat-P satellite system requires that the standard data rates up to 4800 bit/s (e.g. 1200, 2400, 4800 bit/s) can be transmitted on one traffic channel, and that the standard data rates exceeding 4800 bit/s (e.g. 9600, 14400, 19200 bit/s, etc.) can be transmitted by using several parallel traffic channels, like in the HSCSD service of the GSM system.
In the Inmarsat-P satellite system, the data rate of one traffic channel at the radio interface is at most 4800 bit/s, which equals the user data rate of 4800 bit/s at the terminal interface. In a data service employing two traffic channels, the data rate at the radio interface equals the user data rate of 9600 bit/s at the terminal interface. A problem arises when not only the user data but also the above-described terminal interface status information and any inter-subchannel synchronization information should be transmitted over the radio interface. The protocol data unit, i.e. frame structure, used by the satellite system at the radio interface should therefore be defined to carry the above-mentioned control and synchronization information over the radio interface.
One approach would be to use a GSM solution, i.e. a V.110-based frame structure, also at the radio interface of the satellite system. However, this would be a very complicated solution, and it would significantly reduce the user data rates available. A single traffic channel could not support the user data rate of 4800 bit/s since a V.110 frame structure and the terminal interface status information raise the actual data rate (radio interface rate) to be higher than 4800 bit/s. Therefore the highest standard user data rate on one traffic channel would be 2400 bit/s. For the same reason, a two-traffic-channel data service could not support the user rate of 9600 bit/s, but the highest standard user data rate would be 4800 bit/s (or in some systems 7200 bit/s). A corresponding decrease in the available data rates would also occur in data services employing more than two traffic channels. Such a solution, where the overhead information causes a significant loss of capacity, would not be satisfactory.
A similar problem can also arise when other types of radio interfaces, such as wireless telephone systems, are connected to the PLMNs.
A similar problem can also arise on other types of connections in which the radio interface rate is to be used as efficiently as possible. For example, a new 14400 bit/s traffic channel has been planned for the GSM. In order that the terminal interface statuses and any other control information could be transmitted over the radio path in addition to the 14400 bit/s user data, the radio interface rate, implemented on the present principles, must be higher than 14400 bit/s, about 18 kbit/s. A higher radio interface rate requires that the existing radio networks should be re-designed and the intermediate rate (TRAU) raised so that only two subchannels could be put in one 64 kbit/s timeslot in the HSCSD service (i.e. the efficiency decreases in a TRAU data link). A modification of the TRAU frame might make it possible to decrease the intermediate rate to 16 kbit/s, whereby the efficiency of the TRAU data link would not be impaired. The radio interface rate of 14400 kbit/s can be formed, for example, from the present radio interface rate of 12000 kbit/s by enhancing the puncturing that follows channel coding. The radio interface rate of 14400 kbit/s could not, however, transmit the necessary additional information with the user data rate of 14400 kbit/s, but the actual user data rate would be below 14400 kbit/s. The radio interface rate can be slightly raised (e.g. 100 to 300 bit/s) by enhancing the efficiency of the puncturing, and extra bits can be obtained thereby for the transmission of said control information. The enhancement of the puncturing, however, impairs the ability of the channel coding function to correct transmission errors.
In the above-described solutions, control information is transmitted in a frame structure (TRAU, radio burst) outside the user data stream.
Another approach, in which the control information is transmitted inside the user data stream, is disclosed in the Applicant""s parallel patent applications FI 955,496, FI 955,497 and FI 963,455. These applications describe data transmission methods in which the terminal interface status information and any other control or synchronization information are transmitted through a traffic channel in the redundant data elements of end-to-end protocols, such as the redundant parts of the protocol data units of user data or the start and stop bit positions of asynchronous data characters. The overhead information does thus not increase the number of the bits to be transmitted, so the transmission capacity of the traffic channel (e.g. radio interface rate of 14400 kbit/s) can be exactly the same as the user data rate at the terminal interface (e.g. 14400 kbit/s). No additional puncturing is thus needed at the radio interface for the transmission of the control information. In high-rate data transmission (HSDSD) a data link comprises a group of two or more traffic channels, whereby the total capacity of the group of traffic channels can be the same as the user data rate at the terminal interface.
Both the above approaches, however, pose an additional problem.
When the status and control information are transmitted in redundant bits inside the user data stream in the redundant data elements of the end-to-end protocols, then the transmission is dependent on the redundancy of the end-to-end protocols. Not all end-to-end protocols contain a sufficient number of redundant bits for carrying the terminal interface status bits, subchannel numbering bits and NIC code bits. This means that these protocols cannot be supported at all in transparent data transmission.
When the status and control information are transmitted on a traffic channel outside the user""s end-to-end data stream, the transmission of user data is completely transparent, i.e. any end-to-end protocol whatsoever can be used. A problem, however, is that for example in the GSM a TRAU frame is not able to carry the terminal interface status bits, the subchannel numbering bits and the NIC code bits at the intermediate rate of 16 kbit/s. The intermediate rate of 16 kbit/s requires a frame structure that is so compact that there is no room for this additional information. On the other hand, a higher intermediate rate would restrict the number of subchannels in HSCSD transmission, as stated above.
The object of the invention is to eliminate the above problems.
The invention relates to a data transmission method according to claim 10, an equipment according to claim 8, and a mobile system according to claim 15.
The bits available for the transmission of the extra control information, such as the terminal interface status bits, the subchannel numbering bits and the NIC code bits, form a subframe, and two or more subframes form a superframe. In the invention, the information is then multiplexed inside successive subframes in the superframe structure. In other words, the capacity of the bits (subframe bits) available for the transmission of control information is shared in the time domain by various kinds of control information by means of the superframe structure. Preferably one, optionally several such bits in each subframe are used to form a superframe structure, i.e. to indicate at least where the superframe starts and optionally where it ends, and to produce the synchronization information. The remaining subbit or subbits are used to transmit the various kinds of status and control information in multiplexed form inside the superframe thus formed. The superframe bit itself can also be used to transmit the status and control information, if the superframe locking character is shorter than the number of bits reserved for it in the superframe.
The invention allows transmission of terminal interface status and control information and other control information, subchannel and/or frame numbering of a multichannel connection, and NIC codes, even if the number of available bits in one transmission frame or end-to-end user data protocol unit is smaller than the total bit number of the information to be transmitted. The only requirement is that in each frame or each end-to-end user data protocol unit the number M of bits available for this purpose is at least 2, if the superframe bit itself is not to be used or cannot be used for transmission of status or control information. If one and the same bit is used both for superframing and for the transmission of the status and control information, M can be 1. The size of the superframe, i.e. the number L of subframes within the superframe, depends on the total number N of bits to be transmitted and the number M of available transmission bits per subframe, being thus Lxe2x89xa7M/N. Generally, N greater than Mxe2x89xa71 and Lxe2x89xa72.
The invention is equally well suited for the transmission of control information both outside and inside the user data stream.
When data is transmitted in a frame structure (such as TRAU) outside the user data stream, the invention eases the pressure put on the intermediate rate and thereby allows a larger number of subchannels in multichannel data transmission (HSCSD). In addition, the number of additional bits needed at the radio interface (radio interface rate) can be reduced, which in turn decreases the need of additional puncturing.
The invention makes transmission inside the user data stream possible with all end-to-end protocols in which there are at least two bits available in the redundant data elements for the transmission of status and control information.
The term xe2x80x98subframexe2x80x99 is here to be understood in a very general sense. In the invention, a subframe comprises the bits reserved for the transmission of the control information to be multiplexed inside an actual transmission frame or in the redundant data elements of end-to-end protocols, such as the redundant parts of the protocol data units of user data or the start and stop bits of asynchronous data characters. A xe2x80x98superframexe2x80x99 in turn is a unit comprising two or more such successive frames.